Minuscule robots for targeted drug delivery

Such robots will have a long and challenging list of requirements. For example, they need to survive body fluids such as stomach acid and be controllable so they can be guided precisely to their target site. They must also release medical substances only when they reach their target, where they can then be absorbed by the body without causing harm.

Now, a Caltech-led team has developed tiny robots that meet all of these requirements. Using these robots, the team successfully delivered therapy that reduced the size of bladder tumors in mice. A paper describing the work appears in the journal scientific robot.

“We designed a single platform that can solve all of these problems,” said Wei Gao, a professor of medical engineering at Caltech and a researcher at the Institute of Traditional Medicine and co-corresponding author of a new paper on the robot, which the team calls bioabsorbable. Acoustic Microrobot (BAM).

“Instead of putting the drug into the body and letting it spread everywhere, we can now guide our microrobots directly to the tumor site and release the drug in a controlled and effective way,” Gao said.

The concept of micro or nanobots is not new. People have been developing these versions for the past two decades. However, their use in living systems has so far been limited because precisely moving objects in complex biological fluids such as blood, urine or saliva is extremely challenging, Gao said. The robots must also be biocompatible and bioabsorbable, meaning they will not leave any toxic substances behind in the body.

The microrobots developed at Caltech are spherical microstructures made from a hydrogel called polyethylene glycol diacrylate. A hydrogel is a material that starts out as a liquid or resin and becomes a solid when the polymer network within it cross-links or hardens. This structure and composition enable hydrogels to retain large amounts of liquid, making many hydrogels biocompatible. The additive manufacturing manufacturing method also enables the outer sphere to deliver therapeutic cargo to target sites in the body.

To develop the hydrogel formulation and create the microstructure, Gao turned to Julia R. Greer, Ruben F. and Donna Mettler of Caltech. (Donna Mettler) Professor of Materials Science, Mechanics and Medical Engineering and Director of the Fletcher Jones Foundation’s Kaveri Institute for Nanoscience and colleagues sought assistance. Greer’s team has expertise in two-photon polymerization (TPP) lithography, which uses extremely fast infrared laser pulses to selectively cross-link photopolymers in a very precise manner based on specific patterns. The technology allows structures to be built layer by layer, similar to a 3D printer, but in this case with greater precision and formal complexity.

Greer’s team successfully “wrote” or printed microstructures about 30 microns in diameter, about the diameter of a human hair.

“This particular shape, this sphere, was very complicated to write,” Greer said. “You have to know certain tricks of the trade to keep the spheres from collapsing in on themselves. Not only were we able to synthesize resins that contained all the biofunctionalization and all the medically necessary elements, but we were able to write them in a precise way into a sphere with the necessary cavities.”

In its final form, the microrobot combines magnetic nanoparticles and therapeutic drugs within the outer structure of the sphere. Magnetic nanoparticles allow scientists to use external magnetic fields to guide robots to desired locations. When the robots reach their target, they stay at that location and the drug passively diffuses out.

Gao and colleagues designed the outside of the microstructure to be hydrophilic, or attracted to water, which ensures that individual robots do not clump together as they move through the body. However, the microrobot’s inner surface cannot be hydrophilic because it needs to trap air bubbles, which can easily burst or dissolve.

To construct hybrid microrobots that are hydrophilic on the outside and hydrophobic or water-repellent on the inside, the researchers designed a two-step chemical modification. First, they attached long-chain carbon molecules to the hydrogel, making the entire structure hydrophobic. The researchers then used a technique called oxygen plasma etching to remove some of the long-chain carbon structures from the inside, making the outside hydrophobic and the inside hydrophilic.

“This is one of the key innovations of the program,” said Gao, who is also a Ronald and Joanne Willens Scholar. “This asymmetric surface modification, hydrophobic on the inside and hydrophilic on the outside, does allow us to use many robots and still capture bubbles in biological fluids such as urine or serum for long periods of time.”

In fact, the team showed that with this treatment, the bubbles can last for days, whereas other treatments only last a few minutes.

The presence of trapped bubbles is also crucial for moving the robot and tracking them via real-time imaging. For example, to achieve propulsion, the team designed the microrobot sphere to have two cylindrical openings—one at the top and another on the side. When the robot is exposed to an ultrasonic field, the bubbles vibrate, causing surrounding fluid to flow out of the robot through the openings, pushing the robot through the fluid. Gao’s team found that using two openings allowed the robot to not only move through a variety of viscous biological fluids, but also move at higher speeds than a single opening.

Each microstructure contains an egg-shaped bubble that serves as an excellent ultrasound imaging contrast agent, allowing for instant monitoring of the robot. live. With the help of ultrasound imaging expert Mikhail Shapiro, a professor of chemical engineering and medical engineering at Caltech and a Howard Hughes Medical Institute investigator, the team developed a way to track the microrobot as it moves to its target. Co-corresponding author Wu Di, research scientist and director of the DeepMIC Center at Caltech; co-corresponding author Zhou Qifa, professor of ophthalmology and biomedical engineering at the University of Southern California.

The final stage of development involves testing the microrobot as a drug delivery vehicle in mice with bladder tumors. The researchers found that four treatments delivered by microrobots were more effective in shrinking tumors over 21 days than treatments delivered without robots.

“We think this is a very promising platform for drug delivery and precision surgery,” Gao said. “Going forward, we could evaluate using this robot as a platform to deliver different types of therapeutic payloads or agents for different conditions. In the long term, we hope to test this in humans.”

This work was supported by grants from Caltech’s Kaveri Nanoscience Institute and the National Science Foundation; Heritage Institute of Medicine; Singapore Ministry of Education Academic Research Fund; U.S. National Institutes of Health; Army Research Office through Collaborative Biotechnology Institute; the Caltech DeepMIC Center, with support from Caltech’s Beckman Institute and the Arnold and Mabel Beckman Foundation; and the David and Lucile Packard Foundation.